Genetic variation and population differentiation in the forest herb Lathyrus vernus (Fabaceae)

Abstract: In the present thesis, I investigated patterns of genetic variation in the forest herb Lathyrus vernus (Fabaceae), using plant material from different-sized populations representing three geographically separated regions: central Europe, southern and central Sweden. My studies included presumably neutral or weakly selected characters (allozymes and leaflet shape) as well as characters that should be under strong directional selection (e.g. flower and fruit production). The observed patterns were related to geographic distribution (marginality) and population size. Multivariate analyses of allozyme data showed a clear latitudinal pattern, grouping the populations into clusters that corresponded with the three geographical regions. Leaflet shape showed the same regional pattern as the allozymes, but had larger between-population and between-region components in the hierarchical analyses, whereas variation in characters closely connected with fitness was unrelated to allozyme variation, regardless of whether the analyses involved geographic patterns or hierarchical structure. In general, the results indicate a more consistent effect of marginality on allele frequencies at allozyme loci and on mean leaflet shape than on reproductive characters, a possible reflection of differences in the strength of natural selection and genetic drift affecting different characters. I found no significant difference in levels of allozyme variation between small and large populations or between central and marginal populations, and there was no tendency for reproductive performance variables to covary with population size. Further, estimates of inbreeding depression provided no evidence for a long history of purging selection in small or marginal population of L. vernus.